The communication of digital signals along any transmission path results in attenuation through conventional IR losses and in waveform distortion resulting from a filter effect by the transmission path. Additionally, impedance mismatches arising from mixed wire gauges and improper terminations result in echoes or reflections that blur the normally clear distinctions between ones and zeros in digital technology. Thus, in an AMI bit stream a digital zero following a digital +/- one may, on occasion, not reach a zero value due to reflected signals that may occupy the zero time slot. The difficulty of distinguishing between ones and zeros of a bit stream is referred to as intersymbol interference (ISI) which, under severe interference conditions, is commonly referred to as "closing of the eye". Conversely, reopening the eye refers to suppressing the ISI through various means known to those skilled in the art of telephony.
A discussion of the foregoing problem and a solution thereto using an adaptive equalizer is detailed in U.S. Pat. No. 4,545,060 Arnon which issued Oct. 1, 1985. Another form of adaptive equalizer is disclosed in U.S. Pat. No. 4,650,930 Hogeboom et al which issued Mar. 17, 1987.
Generally, the problems associated with ISI are exacerbated through a reduced signal-to-noise ratio, particularly where reflected signal strengths are large. U.S. Pat. No. 4,679,209 Hogeboom et al, issued July 7, 1987, discusses this problem and proposes a digital line receiver that is adapted to provide an improved signal-to-noise ratio of about 6 dB.
Adaptive equalizers nd digital line receivers, as typified by the aforenoted patents, comprise sophisticated line receiver designs in which an adaptive forward filter/equalizer reduces the noise bandwidth in a controlled manner whereas ISI is suppressed by an adaptive decision feedback equalizer (DFE) so as to reopen the eye after being initially closed by the filter.
In addressing the perceived problems of attenuation and ISI, other problems are introduced. One relates to timing recovery for establishing a clock at the receive end of the telephone line. Due to the high distortion intentionally generated by the filter of the prior art, it is not feasible to simply extract timing from the bit stream without engaging some complicated means of channel estimation or by transmitting special sequences or tones in the bit stream to aid timing recovery.
Another problem relates to the DFE which, because of the high distortion introduced by the filter, makes convergence of the DFE uncertain. A special protocol or a predetermined sequence of bits is required to initialize the system. Accordingly, any signal interruption requires a complete initialization procedure in order to be restarted.
As a result of the aforedescribed problems, the digital line receivers of the prior art require a high degree of intelligence which results in increased circuit complexity and inefficient utilization of transmission bandwidth.